A AlGaN/GaN heterojunction field-effect transistor using a GaN layer as an electron transit layer is known. GaN is a wide bandgap material with high breakdown voltage and high saturation electron velocity. GaN is a promising material that may realize a high-current, high-voltage and low ON-resistance semiconductor device. Accordingly, many studies and much research have been conducted on a GaN-based semiconductor device as the next-generation high-performance switching device.
In general, in a semiconductor device such as a field-effect transistor, an insulating film is formed over the entire surface of the devices (such as the field-effect transistors) for the purpose of passivation after the gate electrodes or the drain electrodes are fabricated.
To realize a high-performance switching device using a power transistor, it is desired to reduce the ON-resistance, while realizing normally-off behavior and high breakdown voltage of the switching device. Low ON-resistance and Normally-off behavior can be realized by improving the GaN crystal quality and/or improving the crystal qualities of the materials used in the transistors. On the other hand, it is in general difficult for a switching device using a Schottky gate structure to realize a high breakdown voltage because dielectric strength of several hundreds volts to several kilovolts is required depending on applications. To overcome this issue, it is proposed to insert an insulating film between a gate electrode and a semiconductor layer to reduce gate leakage current and enhance the dielectric strength.
A protection film or a passivation film of an insulation material is also provided to a transistor with an insulating film inserted between the gate electrode and the semiconductor layer. However, the protection film may lower the dielectric strength of the transistor, and as a result, a sufficient level of dielectric strength may not be achieved.
Accordingly, it is desired to achieve sufficient dielectric strength in a semiconductor device, such as a transistor, with an insulating film provided between a gate electrode and a semiconductor layer.